The present invention relates, in one aspect, to a method for enhancing at least one of the mechanical properties of a partially crystalline thermoplastic, by dispersing within said partially crystalline thermoplastic a polydiorganosiloxane of a particular description, and extending the resulting dispersion in a forming step. Ultimate tensile strength, ultimate elongation, and/or modulus can be increased in value by the method of the present invention. In another aspect, the present invention relates to thermoplastic articles having at least one enhanced mechanical property obtained through the method of the present invention.
One of the primary problems to which applied polymer science has addressed itself has been the problem of enhancing the mechanical properties of thermoplastic polymers. Efforts addressed to this problem have resulted in novel polymers, new fillers, new crosslinkers, and new methods of fabrication. These efforts have been productive. Whereas thirty years ago the term "plastic" was almost synonomous in the public mind with fragile, today certain thermoplastics can be confidently substituted for metals in many applications. The advantages gained in ease of fabrication, weight efficiency, and corrosion resistance are well known. However, there still exists a need to enhance the mechanical properties of thermoplastics, especially to enhance the mechanical properties of such thermoplastics as polyolefins and polyesters, which are less expensive, but also less strong than such exotic high strength thermoplastics as polyimides and the like.
Silicones of various descriptions have been incorporated into thermoplastic compositions for the purpose of modifying surface properties or bulk physical properties.
U.S. Pat. No. 3,087,908 discloses the incorporation of up to about 400 parts per million of organopolysiloxane into polycarbonates for the purpose of improving film-forming and fiber-forming properties. The resulting mixture of organopolysiloxane and polycarbonate can be fabricated into objects more free from surface physical abnormalities, such as craters, bubbles, and fisheyes, than objects fabricated of polycarbonate with no added organopolysiloxane.
U.K. Pat. No. 1,428,331 discloses the incorporation of either a polydiorganosiloxane, or an inert powder such as talc, into a thermoplastic polyester. The subsequent mixture was extended to 2.8 to 3.7 times its original longitudinal dimension. The extended mixture had a decreased coefficient of friction, and was incorporated in a laminate of two sheets. Said laminate was found to have enhanced adhesion between the sheets.
Japanese patent application No. 49-30873 discloses the preparation of polyethylene napthalate, and copolymers thereof, with a polydiorganosiloxane present in the reaction medium during a polycondensation reaction. The benefits conferred by the polydiorganosiloxane are disclosed as improved polymerization yield and superior molding properties. Fibers were spun from the product of the polycondensation reaction, and said fibers were drawn over a hot pin to an extension 4 times their original longitudinal dimension.
U.S. Pat. No. 3,842,153 discloses the use of certain organosilicon compounds to lower the melt viscosity of polypropylene by fostering degradation of the polypropylene.
U.S. Pat. No. 4,287,108 discloses the incorporation of certain polydiorganosiloxanes into polypropylene for the purpose of enhancing the elastic properties of the polypropylene. Specifically, said incorporation produces polypropylene articles having a lower value of permanent elongation than polypropylene with no polydiorganosiloxane incorporated therein.
U.S. Pat. No. 3,253,506 discloses the use of crosslinkable organosiloxane compositions to immobilize an extended linear organic polymer for the purpose of rendering more permanent an oriented matrix for a dichroic material.
While the references cited above disclose a variety of modifications to thermoplastics accomplished by incorporation of various silicones, said references rely on imparting to the thermoplastic polymer one or more of the recognized attributes of silicones, such as slipperiness, fluidity, surface smoothness, elasticity, ability to be crosslinked, and other properties commonly associated with silicones.
None of the references cited above disclose enhancement of the mechanical properties, such as ultimate tensile strength or modulus, of the thermoplastic into which the silicones were incorporated.
Enhancement of mechanical properties, as indicated by increases in ultimate tensile strength, and/or increases in stiffness, as indicated by increases in modulus, are not only unexpected upon incorporation of a liquid silicone in a solid thermoplastic, such increases are contrary to what would normally be expected when incorporating a liquid in a solid thermoplastic. Incorporating a liquid in a solid thermoplastic would normally be expected to result in a more rubbery composition, characterized by lower modulus and/or lower tensile strength. A well known example of incorporating a liquid in a solid thermoplastic is adding a plasticizer to a thermoplastic to render said thermoplastic softer and more rubbery.